When adapting fairytales for modern tastes, the trend has been to go one of two ways: knowing, postmodern-lite comedy, or knowing, postmodern-regular horror. Mirror, Mirror, a retelling of Snow White starring Julia Roberts, has taken the first route. Later in the year, Kristen Stewart will Twilight up a darker rendering of the story in Snow White and the Huntsman.

The conventions of Once Upon a Time and Happily Ever After have led to fairytales being misunderstood as sappy little fables with no bearing on modern life, which need to be either teased or mutilated to please a contemporary audience. We say, "Life's not a fairytale" when trying to snap someone out of a romanticised or over-optimistic outlook, but this reveals a fundamental ignorance of fairytales, probably influenced by Disney's cleaned up, universally familiar and gorgeous, versions. I love Disney and would prefer to watch the cute version of The Little Mermaid, bleached of the impassive horror of the original, but the cruelty of Hans Christian Anderson's story fits a genre where eyes are pecked out, toes sawn off, and drownings and immolations are commonplace.

It is generally accepted that fairytales introduce children to the dark side of life while keeping the horror at a safe distance. Any threat should be fantastical and impossible, and confined to an absorbing and tangentially familiar storyline. When I was little my mum would read me Hansel and Gretel as a bedtime story; it always made me ask for bread and butter, but it didn't make me think she would take me into the woods and leave me there. However, the lesson to take stones in your pocket, rather than breadcrumbs, is that you should make the best of what is available to you but that sometimes this isn't enough. If there are only breadcrumbs and the birds eat them, your planning and ingenuity will be scuppered. Still, Hansel had the sense to poke a stick through the bars of the cage to fool the stupid witch, he kept his wits and finally outfoxed the grownups.

Fairytales teach us to trust old women, apart from when we can't trust them; that beauty equals goodness, except when it is a symbol of pure evil. Generally, the only truly admirable characters in fairytales are children, and then they have to rely on their guile (and a little bit of magic) to win out.

Another of my favourites is the Baba Yaga story, a Russian fairytale about a witch who kidnaps a little girl. In order to escape the girl has to use the only weapons available to her: the contents of her purse. She throws behind her a comb, which turns into a forest, a mirror, which turns into a river, and a third object, often a needle, which tears the earth into an impassable gulf. I love how these ordinary domestic objects become whipped up into astonishing natural phenomena, which seems a pretty good metaphor for childhood's habit of inflating into enormity small events, objects and time spans.

As adults we remain affected by the lessons we have learned from fairytales. Rather than using a narrative frame to guide us in our movements, we bend our experiences to fit the storyline – whichever comedy or gothic horror, or romance best suits our needs at the time. Thus, with hindsight, we recast friends, lovers, family as the wicked witch, the true love, the ingénue. A love affair can be beset with disasters, coincidences, chaos and lost slippers, and if it works out it is cast as "meant to be". If it fails, it was clearly a disaster waiting to happen.

Our heroes switch places with our villains with every family feud and unveiled secret. Our successes and our failures become inevitable once they are behind us. The longer we linger, the more complicated Once Upon a Time becomes, until we feel that we've lived several existences and been several characters. Sometimes the people we have been knit nicely together, other times they clunk together uneasily like disparate charms on a bracelet. We parcel our lives up into segments, in which the moment – this very second – is The End.